This invention relates generally to improved sealing means for use in turbo-machines and the like. More particularly, but not by way of limitation, this invention relates to an improved sealing means and a control system operable therewith that provides both a fluid seal with the turbo-machine rotor and with the housing and provides a variable shaft support that, in effect, provides a variable amount of stiffness to the rotor shaft to provide higher critical speeds during operation of the turbo-machine.
The desirability for providing a plurality of stages in turbo-machinery has lead to the use of relatively long shafts. In such turbo-machinery, it is desirable to maintain the shaft diameter as small as possible to reduce the inertia of the rotating parts of the machine and, with relatively long shafts in multi-stage machines, considerable difficulty has been encountered due to the vibrations that are induced when using the relatively long flexible shaft. Manifestly, it is necessary to maintain shaft stiffness to reduce the vibrations as the rotative speed passes through the critical speeds of the shaft. In view of the present day high shaft operating speed, it is highly desirable to increase the critical speed as much as possible. To further complicate the matter, it is not practical in a turbo-machine to install shaft supporting bearings between the bearings located at each end of the shaft housing.
It has been found that the shaft stiffness can be increased to some extent by the use of seals that are biased by fluid under pressure against the housing so that frictional engagement between the housing and seal occurs. Such forces resist lateral or radial motion of the shaft as it is rotated until the frictional force generated by the pressure is exceeded. Seals can, of course, be designed that provide the optimum of frictional force for known conditions. However, some unpredictable forces frequently enter into the rotor dynamics during operation which cannot be completely anticipated in the original design. In such circumstances, it is highly desirable to be able to vary the amount of frictional force exerted while the turbo-machine is in operation to overcome the unpredicted forces.
This invention provides an improved shaft stiffness control apparatus that functions effectively as a seal and provides a variable amount of frictional engagement to overcome the unpredicted rotor dynamic loading.